__canary__ is now in Puppet

Since https://github.gds/gds/puppet/pull/2288, the __canary__ has been in
the Nginx configuration in Puppet. We don't need this file as well as that
one, as the Nginx configuration is serving the content (and setting it to be
JSON, much like the __canary__ on GOV.UK).

Removes the file to avoid confusion. Cc: @dancarley
Merge pull request #477 from alphagov/remove-canary

__canary__ is now in Puppet
Optimises PNGs
Merge pull request #478 from alxndrsn/optimise_pngs

Optimises PNGs
Add image_optim gem
Run all of app/assets through image_optim
Updates README and CONTRIBUTING ref image optimisation
Merge pull request #479 from alxndrsn/image_optim

Image optim
Make image_optim development group only

We don't need this Gem to compile assets, it's for convenience only.
Merge pull request #480 from alphagov/make-image-optim-development-group

Make image_optim development group only
Move global rpint styles to static.

• Hide breadcrumbs
• fix cross browser exclamamtion mark for info-notice and help-notice
Merge pull request #481 from alphagov/update-print-styles

Move global print styles to static.
Bump govuk_template to latest version (0.10.0)

This will update the OGL link & text to refer to version 3.
Merge pull request #482 from alphagov/bump_govuktemplate_to_0100

Bump govuk_template to latest version (0.10.0)